1. Technical Field
The present invention relates to wireless communications, and more particularly, to a method of transmitting reference signals in a cooperative wireless communication system (i.e., multi-cell cooperative wireless communication system or a relay-based wireless communication system) and a method of allocating radio resources (RRs) in the relay-based wireless communication system.
2. Related Art
In next generation multimedia mobile communication systems, which have been actively studied in recent years, there is a demand for a system capable of processing and transmitting a variety of information (e.g., video and wireless data) in addition to an early-stage voice service. In order to maximize efficiency of a limited radio resource in a mobile communication system, methods for more effectively transmitting data in time, space, and frequency domains have been proposed.
Orthogonal frequency division multiplexing (OFDM) uses a plurality of orthogonal subcarriers. Further, the OFDM uses an orthogonality between inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT). A transmitter transmits data by performing IFFT. A receiver restores original data by performing FFT on a received signal. The transmitter uses IFFT to combine the plurality of subcarriers, and the receiver uses FFT to split the plurality of subcarriers. According to the OFDM, complexity of the receiver can be reduced in a frequency selective fading environment of a broadband channel, and spectral efficiency can be increased when selective scheduling is performed in a frequency domain by using a channel characteristic which is different from one subcarrier to another. Orthogonal frequency division multiple access (OFDMA) is an OFDM-based multiple access scheme. According to the OFDMA, efficiency of radio resources can be increased by allocating different subcarriers to multiple users.
To maximize efficiency in the space domain, the OFDM/OFDMA-based system uses a multiple-antenna technique (e.g., multiple input multiple output (MIMO), single input multiple output (SIMO), and multiple input single output (MISO)) which is used as a suitable technique for high-speed multimedia data transmission by generating a plurality of time/frequency domains in the spatial domain. The OFDM/OFDMA-based system also uses a channel coding scheme for effective use of resources in the time domain, a scheduling scheme which uses a channel selective characteristic of a plurality of users, a hybrid automatic repeat request (HARQ) scheme suitable for packet data transmission, etc.
To guarantee high-speed data transmission, each channel through which data is transmitted has to be estimated with high reliability. It is important to design a reference signal for channel estimation in order to increase the reliability of channel estimation. The reference signal is known, in advance, to both the transmitter and the receiver, and is also referred to as a pilot. A channel environment may change variously depending on time, space, and frequency, and thus the reference signal has to be transmitted to the receiver by designing the reference signal such that the reliability of channel estimation can be increased by flexibly coping with the channel environment.
FIG. 1 shows an example of a frame structure, in particular, a frame structure for a long term evolution (LTE) wireless communication system. A ‘frame’ is a data sequence used according to a physical specification in a fixed time duration, and is an arbitrary time unit for processing data in a wireless communication system. The term ‘frame’ is for exemplary purposes only, and thus its term and structure may differ depending on a type of the wireless communication system.
Referring to FIG. 1, the frame for the LTE wireless communication system consists of 10 subframes. Since one frame has a length of 10 milliseconds (ms), a subframe has a length of 1 ms. One subframe may include a plurality of OFDM symbols (i.e., 14 OFDM symbols) in a time domain, and may include any number of subcarriers in a frequency domain. As such, the subframe is time-spanned on the OFDM symbols, and is frequency-spanned on the subcarriers. One subframe may include a plurality of basic units. Hereinafter, a basic unit denotes an allocation unit of reference signals. The basic unit is time-spanned on the OFDM symbols, and is frequency-spanned on the subcarriers. The basic units can be consecutively assigned in the time domain and/or the frequency domain for data transmission. Preferably, the number of OFDM symbols for time-spanning the basic unit is set to a divisor of the number of OFDM symbols for time-spanning the subframe. For example, if the subframe is time-spanned on 6 OFDM symbols, the basic unit may be time-spanned on one OFDM symbol, two OFDM symbols, three OFDM symbols, or six OFDM symbols.
A ‘resource block’ is a term used in association with the basic unit in the LTE wireless communication system. The resource block is a unit of allocating radio resources to a user. In the LTE wireless communication system, one resource block may consist of 12 subcarriers by 7 OFDM symbols.
The aforementioned frame structure is for exemplary purposes only, and thus the number of subframes included in the frame, the number of OFDM symbols for time-spanning the subframe, the number of subcarriers for frequency-spanning the subframe, a structure of a basic unit included in the subframe, or the number of basic units included in the subframe may variously change.
In general, reference signals are periodically transmitted in the wireless communication system. In particular, according to a conventional method of transmitting reference signals, the reference signals are designed to have a single transmission pattern in the aforementioned frame structure. For example, when one frame consists of several subframes, specific positions of each subframe are assigned for reference signals by each base station (BS), and the BS transmits the reference signals arranged in the same pattern to a mobile station (MS).
The conventional method of transmitting reference signals is used for a conventional wireless communication system, i.e., a non-cooperative wireless communication system in which one MS communicates with only one BS without cooperative transmission and reception between cells and/or in which a relay station (RS) is not used in communication between the MS and the BS. In the non-cooperative communication system, no problem has arisen in terms of communication between the MS and the BS or efficient usage of radio resources, even if the reference signals are transmitted based on only one pattern. A cooperative communication scheme of a plurality of BSs and/or a multi-hop transmission scheme using one or more RSs are considered in a newly proposed wireless communication system of next generation. However, the aforementioned conventional method of transmitting reference signals may experience difficulty in effectively operating communication/transmission schemes to be newly introduced in a cooperative wireless communication system.
Accordingly, there is a need for a new method of transmitting reference signals so as to effectively operate communication/transmission schemes to be newly introduced in a cooperative wireless communication system.